Electromyography (EMG) Instrumentation

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Electromyography (EMG) Instrumentation

• David Groh
• University of Nevada Las Vegas

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Research Applications of Surface EMG

• Indicator for muscle activation/deactivation
• Relationship of force/EMG signal
• Use of EMG signal as a fatigue index

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Types of EMG

• Electrode Categories
• Inserted
• Fine-wire (Intra-muscular)
• Needle
• Surface

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Fine-wire Electrodes

• Advantages
• Extremely sensitive
• Record single muscle activity
• Access to deep musculature
• Little cross-talk concern
• Disadvantages
• Extremely sensitive
• Requires medical personnel, certification
• Repositioning nearly impossible
• Detection area may not be representative of
  entire muscle

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Surface Electrodes

• Advantages
• Quick, easy to apply
• No medical supervision, required certification
• Minimal discomfort
• Disadvantages
• Generally used only for superficial muscles
• Cross-talk concerns
• No standard electrode placement
• May affect movement patterns of subject
• Limitations with recording dynamic muscle activity

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Electrode Comparison Studies

• Giroux Lamontagne - Electromyogr. Clin.
  Neurophysiol., 1990
• Purpose to compare EMG surface electrodes and
  intramuscular wire electrodes for isometric and
  dynamic contractions
• Results
• No significant difference in either isometric or
  dynamic conditions
• However dynamic activity was not very dynamic

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EMG Manufacturers

• Noraxon
• Motion Lab Systems
• Delsys

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General Concerns

• Signal-to-noise ratio
• Ratio of energy of EMG signal divided by energy
  of noise signal
• Distortion of the signal
• EMG signal should be altered as minimally as
  possible for accurate representation

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Characteristics of EMG Signal

• Amplitude range 010 mV (5 to -5) prior to
  amplification
• Useable energy Range of 0 - 500 Hz
• Dominant energy 50 150 Hz

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Characteristics of Electrical Noise

• Inherent noise in electronics equipment
• Ambient noise
• Motion artifact
• Inherent instability of signal

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Inherent Noise in Electronics Equipment

• Generated by all electronics equipment
• Frequency range 0 several thousand Hz
• Cannot be eliminated
• Reduced by using high quality components

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Ambient Noise

• Electromagnetic radiation sources
• Radio transmission
• Electrical wires
• Fluorescent lights
• Essentially impossible to avoid
• Dominant frequency 60 Hz
• Amplitude 1 3x EMG signal

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Motion Artifact

• Two main sources
• Electrode/skin interface
• Electrode cable
• Reducible by proper circuitry and set-up
• Frequency range 0 20 Hz

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Inherent Instability of Signal

• Amplitude is somewhat random in nature
• Frequency range of 0 20 Hz is especially
  unstable
• Therefore, removal of this range is recommended

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Factors Affecting the EMG Signal

• Carlo De Luca
• Causative Factors direct affect on signal
• Extrinsic electrode structure and placement
• Intrinsic physiological, anatomical,
  biochemical
• Intermediate Factors physical physiological
  phenomena influenced by one or more causative
  factors
• Deterministic Factors influenced by
  intermediate factors

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Factors Affecting the EMG Signal
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Maximizing Quality of EMG Signal

• Signal-to-noise ratio
• Highest amount of information from EMG signal as
  possible
• Minimum amount of noise contamination
• As minimal distortion of EMG signal as possible
• No unnecessary filtering
• No distortion of signal peaks
• No notch filters recommended
• Ex 60 Hz

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Solutions for Signal Interruption Related to
Electrode and Amplifier Design

• Differential amplification
• Reduces electromagnetic radiation noise
• Dual electrodes
• Electrode stability
• Time for chemical reaction to stabilize
• Important factors electrode movement,
  perspiration, humidity changes
• Improved quality of electrodes
• Less need for skin abrasion, hair removal

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Differential Amplification

• Ambient (electromagnetic) noise is constant
• System subtracts two signals
• Resultant difference is amplified
• Double differential technique

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Electrode Configuration

• Length of electrodes
• of included fibers vs. increased noise
• Delsys 1 cm
• Noraxon - ?
• Distance between electrodes
• Increased amplitude vs. misaligning electrodes,
  Multiple motor unit action potentials (MUAP)
• Muscle fibers of motor units are distributed
  evenly, thus large muscle coverage is not
  necessary
• (De Luca).
• Delsys 1 cm
• Noraxon 2 cm?

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Electrode Placement

• Away from motor point
• MUAP traveling in opposite directions
• Simultaneous () (-) APs
• Resultant increased frequency components
• More jagged signal
• Middle of muscle belly is generally accepted

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Electrode Placement

• Away from tendon
• Fewer, thinner muscle fibers
• Closer to other muscle origins, insertions
• More susceptible to cross-talk
• Away from outer edge of muscle
• Closer to other musculature
• Orientation parallel to muscle fibers
• More accurate conduction velocity
• Increased probability of detecting same signal

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EMG Electrode Placement
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Surface Electrode Placement
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Reference Electrode Placement(Ground)

• As far away as possible from recording electrodes
• Electrically neutral tissue
• Bony prominence
• Good electrical contact
• Larger size
• Good adhesive properties

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Off to the Lab!
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References

• Basmajian JV, De Luca CJ. Muscles Alive their
  functions revealed by electromyography (fifth
  ed.). Williams Wilkins, Baltimore, Maryland,
  1985
• Cram JR, Kasman GS. Introduction to surface
  electromyography. Aspen Publishers, Inc.
  Gaithersburg, Maryland, 1998
• De Luca CJ Surface electromyography detection
  and recording. DelSys, Inc., 2002
• De Luca CJ The use of surface electromyography
  in biomechanics. J App Biomech 13 135-163, 1997
• MyoResearch software for the EMG professional.
  Scottsdale, Arizona, Noraxon USA, 1996-1999